Evidence for slab break-off in subduction zones

The detachment of lithospheric masses and their falling-off into the mantle, either by delamination of deep lithosphere beneath continents or the breaking of a subducted slab, have become popular means of explaining a variety of unusual phenomena in mountain belts. In the Himalaya and Tibetan Plateau, such models have been evoked for the formation of odd K-rich basalts in the Eocene and Miocene, and the crustal melting that generated leucogranites around 20 Ma ago along the entire length of the Greater Himalaya. Taking all the oddities of the Indo-Asian collision zone together does seem to support such a model (Kohn, M.J. & Parkinson, C.D. 2002. Petrologic case for Eocene slab breakoff during the Indo-Asian collision. Geology, v. 30, p. 591-594). However, there is still no tangible direct evidence beneath the region.

Using seismograms for deep-Earth tomography appears to be able to resolve a range of proposed variants of tectonics, as well as the gross behaviour of the deep mantle. The site where two plates are being subducted on the west side of the North Pacific, marked by the Kamchatka peninsula, is pretty odd as well. Although rates of subduction of both plates are high, the part of Kamchatka at one boundary no longer has active volcanoes, whereas the other does. In fact one of the volcanoes there holds the world record for magma output. Up to 5 Ma ago, the whole of Kamchatka was actively volcanic. An explanation for the sudden halt to volcanism is that the dehydrating slab which provides the essential watery fluid for partial melting of the overlying mantle wedge – the source of subduction-zone magmas – broke away from the subduction zone and “fell” into the mantle 5 Ma ago. That would have removed the source of hydrous fluid at a stroke. Seismic tomography now seems to be capable of resolving just such a foundered slab (Levin, V. et al. 2002. Seismic evidence for catastrophic slab loss beneath Kamchatka. Nature, v. 418, p. 763-767). There is no slab beneath the presently inactive volcanoes, whereas it is intact beneath the active ones. The authors also claim that the seismic structure reveals a more recently foundered piece of lithosphere, whose rapid loss of hydrous fluid helps explain the phenomenally high magma production of the Klyuchevskoy volcano. Such slab break-off is clearly a potential engine for enormous changes in magmatism, and the first seismic evidence for it is bound to spur a search for more examples.

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